Selecting I/O Units

Selecting and communicating with I/O units. The I/O unit physical layer interface. Understanding the wide perspective.
Putting It Together

Now that I've introduced the terminology and discussed features important to selecting an I/O system, let's briefly summarize how to put it all together. The following outline describes how I approach the selection and implementation of I/O systems. I've found that these steps may occur in parallel and may be iterative in nature, with decisions being refined and becoming more accurate with each pass. Designing the system requires a significant amount of patience and persistence as well as a flair for precise detail.

  • Inventory the sensors and effectors the system specification requires. Include location, function and details of the devices. Spreadsheets are excellent tools for this documentation.

  • Determine accuracy and resolution requirements. Determine data rate and response-time requirements.

  • Estimate the expanse of the system. Chart the potential communication paths; require blueprints if necessary.

  • Evaluate I/O unit options to fulfill the points, expanse and throughput requirements.

  • Consider modularity not only as a flexible means of communication but also as a means of simplifying maintenance over the system's lifetime.

  • Select the communication infrastructure. Refine the system expanse and the communication paths.

  • Evaluate software support for each I/O unit considered for the system.

  • Start wiring!

Bryce Nakatani ( is an engineer at Opto 22, a manufacturer of automation components in Temecula, California. He specializes in real-time controls, software design, analog and digital design, network architecture and instrumentation.